This invention relates to methods of measuring the dew point temperature of a moist gas, and apparatus suitable for carrying out such measurements.
The temperature at which a sample of a moist gas will become saturated and condense to form water droplets or, alternatively, frost, on a surface with which the moist gas is in contact is known as the "dew point" temperature. Generally, where the dew point temperature is above 0.degree. the moist gas condenses to form water droplets at the dew point temperature, and where the dew point temperature is below 0.degree. C. moist gas condenses to form frost. Although dew points below 0.degree. C. are sometimes referred to as frost points, in the present context the term "dew point temperature" is intended to include dew point temperatures both above and below 0.degree..
It is often important to measure the dew point temperature of a moist gas, for example, one constituting the atmosphere inside a furnace such as a metallurgical furnace used in semiconductor or fabrication processes. The dew point temperature can provide a measure of the water content in the gas which must be kept extremely low in order to protect one or more of the product, a catalyst, and equipment used in the production process against damage e.g. in optical fibre manufacture. Dew point temperature measurements are also important in protecting gas pipelines against corrosion from moisture in the gas supply. Additionally, a dew point temperature measurement of the contents of a cylinder of gas provides a measure of the purity of the gas in the cylinder. In fact, the industrial applications for dew point temperature measuring and dew point temperature measuring apparatus are numerous and varied.
One known method of measuring the dew point temperature of a moist gas involves passing a flow of gas over a mirror surface. A light source, such as a light emitting diode (L.E.D.) is directed towards the mirror surface and a photosensitive detector is positioned so as to detect light, emitted by the L.E.D., which is reflected from the mirror surface. A cooling system is used to reduce progressively the temperature of the mirror surface until moisture in the moist gas condenses to form dew or frost on the mirror surface. Dew (or frost) forming on the mirror surface in the path of light emitted from the L.E.D. causes the light to be deflected (or scattered) from its original path such that the amount of light detected by the photodiode is reduced. By providing a servo loop between the photodiode and the cooling system, the temperature of the mirror may be controlled in order to maintain it at an equilibrium temperature where the rates of condensation and evaporation of water molecules onto or from the mirror surface are equal and a constant mass of water (or frost) is maintained on the mirror thereby providing a constant level of scattering and hence of the light level detected. This occurs at the dew point temperature. This type of apparatus is often referred to as a chilled mirror hygrometer.
However, this method has the disadvantage of requiring a sufficient amount of moisture to condense on the mirror in order to provide a reliably detectable scattering of the light emitted by the L.E.D. Where the moist gas being measured is relatively dry, for example having a moisture content of 10 parts per million of water molecules (or below), it can take a considerable time, typically several minutes, for enough condensation to occur in order to achieve a reduced reading from the photodiode detector. As a result, the progressively reducing temperature of the mirror may have decreased to well below the dew point temperature before a sufficient amount of condensation has occurred. This limits the accuracy with which the dew point temperature an be determined over relatively short periods of time due to the relatively large oscillations of the mirror temperature above and below the dew point temperature before an equilibrium state is reached. Thus, measurements of the dew point temperature can take upwards of one hour to be accurately obtained where the moisture content of the gas is very low, say between ten parts per million and one tenth parts per million (corresponding to dew point temperature between approximately -60.degree. C. and -100.degree. C.). Additionally, problems can occur where dust or other particles settle on the mirror and cause the light from the L.E.D. to be scattered or deflected away from its original path between the L.E.D. and the photodiode detector, leading to inaccurate measurements of the dew point temperature. Furthermore, where, the gas whose moisture content is to be measured itself condenses at a temperature which is above the dew point of the moist gas, (e.g. certain hydrocarbon gases) problems will occur due to the gas molecules condensing on the mirror and deflecting/scattering the light before the mirror even reaches the dew point temperature of the moist gas.
Under certain vapour pressure and temperature conditions pure water cooled below 0.degree. C. can become a supercooled liquid. Another disadvantage of the above-described "chilled mirror hygrometer" method occurs when attempting to measure dew points below 0.degree. C. (i.e. "frost points"). The chilled mirror hygrometer is unable to detect the condensation of individual water molecules which occurs at the frost point temperature when individual water molecules attach themselves to the mirror. Because of their lack of mobility these molecules will have an equilibrium water vapour pressure which corresponds to ice as opposed to super cooled water. The mirror will continue to cool until the water molecules bond themselves into groups of molecules large enough to scatter light. If the groups of molecules arrange themselves into ice or frost crystals then the chilled mirror temperature will eventually reach a state of equilibrium at a temperature corresponding to the frost point temperature of the gas. However the individual water molecules may arrange themselves into groups of super cooled liquid which would give the individual water molecules greater mobility. In the latter case the chilled mirror temperature will eventually stabilise at the dew point temperature which can be several degrees lower than the frost point temperature for a given water vapour pressure. As both pure water and ice are good insulators of electricity and it is therefore not possible to distinguish between pure water and ice by electronic means, the chilled mirror hygrometer operator needs to use a microscope focussed on the mirror to determine whether ice or water has been formed if accurate readings are required when measuring the dew or frost point temperature in clean air or gas.